Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, alcohol and alcohol-containing fuel blends have been recognized as attractive alternative fuels, in particular for automotive applications. Various engine systems may be used with alcohol fuels, utilizing various engine technologies and injection technologies. Further, various approaches may be used to control such alcohol-fuelled engines to take advantage of the charge-cooling effect of the high octane alcohol fuel, in particular to address engine knocking. Engine knocking may also be controlled with other knock control fluids such as water or various water-alcohol mixtures. For example, engine control methods may include adjustment of boost or spark timing in dependence upon the knock control fluid, and various other engine operating conditions.
Engines may be configured with a charge motion control valve (CMCV) or intake manifold runner control (IMRC) valve in the air intake for adjusting an engine burn rate. By adjusting the position of the valve, the flow of air through the valve to a downstream cylinder may be selectively restricted or unrestricted. Engine control systems may be configured to coordinate the positioning of the CMCV with fuel injection based on engine operating conditions.
One example approach for CMCV control is shown by Lewis et al. in US 2007/0119422 A1. Therein, a CMCV is positioned upstream of two port injectors, the two injectors injecting fuels of differing composition into an engine cylinder. The position of the CMCV is adjusted such that the flow of air to the port injector injecting an alcohol fuel is restricted to a greater extent that the port injector injecting a gasoline fuel, under selected engine conditions. Specifically, at higher engine loads, the valve is adjusted to increase air flow and port injection of the alcohol fuel is enabled while at lower engine loads, the valve is adjusted to decrease air flow and port injection of the alcohol fuel is disabled. As such, this helps to reduce the use of spark retard at higher engine loads, while improving engine performance and fuel economy.
However, the inventors herein have recognized a potential issue with such a system. At high engine loads, adjusting the CMCV to increase the flow of air leads to a higher engine burn rate. This, along with the reduced use of spark retard at higher engine loads, can lead to very high cylinder pressures and elevated rates of pressure rise. Such elevated pressure effects can cause structural and NVH issues in the engine. In one example, it may lead to premature engine degradation.
Thus, in one example, some the above issues may be at least partly addressed by a method of operating an engine including a charge motion control valve (CMCV) comprising, at high engine loads, adjusting the CMCV to decrease an engine burn rate while increasing injection of a knock control fluid to address knock.
In one example, an engine may be configured with a CMCV in the engine air intake, upstream of a direct injector configured to inject a knock control fluid into an engine cylinder. As such, by opening the CMCV, the burn rate of the engine may be decreased, thereby reducing cylinder peak pressures and rates of pressure rise, and allowing the engine to operate at higher torque. However, the reduced engine burn rate may also increase a propensity for engine knock, in particular at higher engine loads. Thus, based on engine operating conditions, the CMCV may be opened to reduce an engine burn rate, while an amount of knock control fluid injected may be concomitantly increased to address the knock. In this way, engine performance may be improved.
An engine controller may be configured to adjust whether the CMCV is opened based on the availability of the knock control fluid. Specifically, if knock control fluid is not available, then the CMCV may be closed to increase burn rate and avoid knock, but the engine maximum torque may be limited (e.g. by limiting boost), to avoid high peak cylinder pressure and/or high rate of pressure rise. Conversely, if knock control fluid is available then the CMCV may be opened and the torque limit of the engine may be increased, as the availability of the knock control fluid increases. For example, based on engine operating conditions, a feed-forward likelihood of knock, as well as an amount of knock control fluid required to address the knock may be determined. It may be further determined whether the required amount of knock control fluid is available, for example, as inferred from a fluid level sensor.
In one example, when the amount of knock control fluid available is greater than a threshold (the threshold based on the amount of fluid required to address a feed-forward likelihood of knock), the CMCV may be opened, while increasing injection of the knock control fluid. Herein, by opening the CMCV the burn rate may be decreased to improve engine NVH and reduce engine structural loads. At the same time, the higher likelihood of knock that may result from the lower burn rate may be advantageously addressed with the direct injection of the knock control fluid.
In another example, when the amount of knock control fluid is lower than the threshold, the CMCV may be closed, while adjusting an engine operating parameter, such as boost, to reduce the engine output thereby reducing the likelihood of knock. For example, while the CMCV is closed, engine boost, spark timing, VCT, and/or throttle adjustments may be performed to reduce the likelihood of knock and to keep cylinder pressure within a desired limit (such as, below a structural limit), and further to keep a rate of rise in cylinder pressure within a desired range (such as, below an NVH limit). Additionally, a smaller amount of knock control fluid may be injected. Alternatively, when sufficient knock control fluid is not available, the CMCV may be kept closed, to increase the engine burn rate and address knock with the higher burn rate, and torque adjustments may be made using an alternate engine operating parameter, such as boost, VCT, or spark timing.
The knock control fluid direct injected into the cylinder may include one or more of ethanol, methanol, other alcohols, gasoline, water, and combinations thereof. The amount of fluid direct injected may be based on the composition of the injected fluid. For example, the amount may be adjusted based on the molar composition of the injected fluid. Thus, as a molar ratio of alcohol in the injected fluid increases, the amount of fluid injected may be decreased. The amount may also be adjusted based on a combination of an inherent octane effect, a dilution effect, and an evaporation effect of the injected fluid.
In this way, at high engine loads, a CMCV may be opened to decrease an engine burn rate, and improve engine performance, while increasing the injection of a knock control fluid to address knock arising from the decreased engine burn rate. Additionally, a predetermined amount of spark retard may be maintained. By adjusting whether the CMCV is opened and the engine burn rate is slowed based on the availability of an amount of knock control fluid for addressing the knock at the reduced engine burn rate, engine performance may be improved, in particular at higher engine loads.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.